What are changeable and unchangeable transactions?

To learn if your transactions are changeable or unchangeable, choose Preferences from the Setup menu, then click the Security tab.

If the selection ìTransactions CANíT be Changed; They Must be Reversedî is unmarked, your transactions are changeable. Recorded transactions may be changed or deleted. If you wish to make your transactions unchangeable, mark the selection.

If the selection ìTransactions CANíT be Changed; They Must be Reversedî is marked, your transactions are unchangeable. Recorded transactions must be reversed; they canít be changed or deleted. If you wish to make your transactions changeable, unmark the selection.

What are changeable and unchangeable transactions?